Recently, the IEEE 802.16 standard has attracted public attention as one of wireless communication technologies. The IEEE 802.16 standard is a technology developed as one of schemes for defining a Wireless Metropolitan Area Network (MAN) which provides a wireless communication path between a communication service provider and a user site instead of a telephone line or an optical fiber line, and makes a Metropolitan Area Network (MAN), which is a broadband network for interconnecting, for example, Local Area Networks (LANs) in a city or a certain region, into a wireless network. The IEEE 802.16 standard specifies that one base station can cover, for example, an area of about 50 km radius at a transmission speed of maximum 70 megabits per second.
Currently, an IEEE 802.16d specification (IEEE 802.16-2004) for fixed communications and an IEEE 802.16e specification (IEEE 802.16e-2005) for mobile communications are standardized in the IEEE 802.16 Working Group.
A wireless communication system includes, for example, a base station (BS) and one or more radio terminals (MS: Mobile Station) as user equipment (UE), and transmits or receives a radio signal between a BS and a MS via a radio link. A radio link includes a downlink (DL) which is a direction from a BS to a MS and an uplink (UL) which is the opposite direction.
In the IEEE 802.16e, a radio frame based on an Orthogonal Frequency Division Multiple Access (OFDMA) scheme is used for transmission of a radio signal.
FIG. 14 illustrates an exemplary radio frame of the IEEE 802.16e. In FIG. 14, a transverse axis denotes a time direction indicated by a symbol unit, and a vertical axis denotes a frequency direction indicated by a logical sub-channel (unit grouping a plurality of sub-carriers).
In FIG. 14, a Time Division Duplex (TDD) scheme is used as a multiplexing method of an uplink (UL) and a downlink (DL), and a frame is time-divided into a DL sub-frame of a first half and an UL sub-frame of a second half. A DL sub-frame is a frame which is transmitted to a MS from a BS, and an UL sub-frame is a frame which is transmitted to a BS from a MS.
ADL sub-frame includes fields of a preamble, a Frame Control Header (FCH), a DL-MAP, an UL-MAP, and a DL burst.
A preamble is a known signal pattern (synchronizing signal) used for a MS to detect a BS and to synchronize with a radio frame transmitted by a BS.
A DL-MAP and an UL-MAP (hereinafter, referred as simply “MAP data”) are signals containing allocation information (corresponding MS, modulation scheme used, error correction code, etc.) of a certain radio resource (burst) of a DL sub-frame and an UL sub-frame for a MS. In FIG. 14, six DL bursts of #1 through #6 are allocated to a DL sub-frame. Data (user data or control message) to be transmitted to a MS from a BS can be mapped to a DL burst, and in FIG. 14, an UL-MAP which is one of control messages is mapped to a DL burst #1.
A Frame Control Header (FCH) is a signal which defines information about a BS or information used for a MS to decode a burst (containing MAP data) of a DL sub-frame, and includes information such as a mapping area of a DL-MAP in a DL sub-frame, a coding scheme, the number of repetition times, etc.
Meanwhile, a plurality of UL bursts (five of #1 to #5 in FIG. 14) can be mapped to an UL sub-frame, and data (user data or control message) to be transmitted to a BS from a MS can be mapped to an UL burst. Also, a part of an UL burst can be allocated as a ranging area (ranging sub-channel) used, for example, when a MS attempts an initial access to a BS.
A MS detects a preamble signal of a DL sub-frame to establish synchronization with a radio frame transmitted from a BS, and demodulates and decodes MAP data (DL-MAP and UL-MAP) based on a FCH, thereby recognizing which burst (frequency and timing) in a radio frame can be used in communication with a BS and which modulation/demodulation scheme and error correction code can be used in the communication.
That is, in the IEEE 802.16e, both communications of a DL and an UL are performed according to MAP data generated by a BS. For this reason, in the IEEE 802.16e, an MS is allocated an UL burst for UL data transmission from a BS when a MS desires to transmit UL data. There are several scheduling types as a communication control method for this.
Various applications such as a web, a voice, a motion picture, etc. are executed in a MS. For example, a silence-compressed voice communication application in which a real-time characteristic of data is high and variable-rate traffic is generated is taken as an example.
In a case of such an application, traffic includes a voiced section in which UL data is generated and a silence section in which UL data is not generated, which depend on whether a voice exists or not. In a voiced section, a MS transmits UL data at a certain communication rate according to a voice coding scheme. Therefore, a BS is required to allocate an UL burst to a MS.
Meanwhile, in a silence section, a BS is not required to allocate an UL burst to a MS, and it can use the unused radio resource in a communication with any other MS, so that since use efficiency of a radio resource is increased, the UL data throughput in the whole BS can be increased.
Therefore, in order to efficiently use a radio resource, it is preferred that a BS dynamically or rapidly recognizes an allocation of an UL burst for a MS according to whether UL data from a MS exist or not and then this is reflected in scheduling. As a communication control method suitable for an UL connection with such a traffic characteristic, a scheduling type so called an extended real time polling service (ertPS) is specified in the IEEE 802.16e.
In the ertPS, based on a maximum transmission rate (Max sustained rate) which is one of traffic parameters set when an UL connection is established, a BS can periodically allocate an UL burst to a MS for transmission of UL data without an allocation request from a MS. Also, a BS can perform polling for MSs in consideration of the fact that there is a case where a MS requests to change an allocation size of an UL burst (an allocation is suspended when an allocation size is changed to zero (0)).
Here, in the case of the IEEE 802.16e, “polling” means that a BS periodically allocates an UL burst small enough for a MS to transmit a Bandwidth Request (BR) message (e.g., 6 bytes) which is an allocation request message of an UL burst. In the IEEE 802.16e, when the ertPS is used, a polling cycle is arbitrary. There also exists a BS which does not perform such polling.
When UL data is not generated from an application as in a silence section, a MS requests a BS to suspend an allocation of an UL burst, thereby suspending an allocation of an UL burst from a BS. Thereafter, when UL data is generated in a MS, a MS requests a BS to allocate an UL burst suitable for an UL data size again. As described above, a MS repetitively requests a BS to suspend/resume an allocation of an UL burst depending on whether UL data exist or not.    [Non-Patent Document 1] IEEE 802.16 (TM)-2004    [Non-Patent Document 2] IEEE 802.16e (TM)-2005
In the above-mentioned art, it is not actively discussed which transmission opportunity a MS has to use to perform an allocation request in a case where transmission opportunities of plural kinds are allocated from a BS for an allocation request of an UL burst of a MS. Therefore, according to timings that a MS performs an allocation resumption request of an UL burst, a delay may occur until an UL burst is actually allocated from a BS, thereby deteriorating communication quality of an uplink.